The liquid scintillation technique is extensively used in radioactivity measurements due to its great versatility (Peng, 1977; Fox 1976; Birks 1964; Horrocks 1974).
It is based on the detection of radiation by means of a scintillation cocktail acting as a transducer by converting the disintegration energy in ultraviolet light, which is collected and amplified by means of photomultiplier tubes generating electric pulses.
The conversion is carried out from the initial interaction of the radiation with the main aromatic solvents of the cocktail, transferring its energy by means of excitation, ionization, formation of free radicals and molecular fragments. The excited solvent molecules in electronic states II emit photons in the ultraviolet region, but their yield is usually poor because the emission probability is low, the photon spectral distribution does not fit with the sensitivity range of the photomultiplier tubes, and the emission half-lives are long (tens of ns), facilitating the loss of energy in non-radioactive forms.
Therefore, the cocktail must also include one or more scintillation phosphors which minimize the prior negative effects, increasing the probability of fluorescence, reducing the half-life to a few nanoseconds, and fitting the photon distribution to the range of the photomultiplier tubes.
The electric pulses produced by the photomultiplier tubes are recorded by means of commercial counting equipment or special prototypes, which usually use two or three phototubes, working in a double or triple coincidence manner by means of suitable temporal analysis and amplitude discrimination circuits. As a result, the counting rate is obtained, which is a function of the efficiency of the whole process.
In practice, this technique is applied by means of adding a radioactive sample to a scintillation liquid or cocktail contained within a glass vial (in order to avoid the permeability of plastics), which allows:    fast preparation of the samples,    4π measurement geometry,    absence of self-absorption,    suitability to alpha, beta, gamma or electron capture emitters.
In order to take advantage of these advantages, it is necessary to prevent interferences such as inhomogeneity, chemiluminescence, phosphorescence, microprecipitation, adsorption, and chemical (impurities) or color quenching, all of them related to the scintillation cocktail composition and the effects of which are summarized below.
The samples must be homogeneous, with all the components completely dissolved forming a single phase in order to have an optimum 4π geometry. Chemiluminescence occurs due to chemical reactions between the cocktail components and the sample itself, producing an additional light emission with a very variable duration. Phosphorescence is due to some compounds having a long-lasting photoluminescence. Microprecipitation occurs due to the incompatibility between the radioactive solution and the scintillation cocktail. Adsorption is due to the affinity between the ions contained in the radioactive sample and the active centers of the inner surfaces of the vial. Chemical quenching occurs due to the presence of impurities making the energy-light conversion difficult. Finally, color quenching is due to the insufficient transparency of the scintillation cocktail for the emitted photons.
The scintillation cocktail composition must take into account these possible interferences, whether they are inherent to the cocktail or the result of incorporating the radioactive sample, in order to obtain stable samples for the sufficient time in order to carry out the measurements and controls which may be necessary in that period.
Different patents (U.S. Pat. No. 4,271,035, U.S. Pat. No. 4,443,356, U.S. Pat. No. 4,867,905 and U.S. Pat. No. 5,135,679) disclose scintillation cocktail formulations and compositions, mainly based on one or several organic solvents and one or more dissolved phosphors, as energy-light transducers, and several additives, ionic and non-ionic surfactants, in order to facilitate the emulsification and incorporation of aqueous samples to the organic liquid.
The objective of these formulations is mainly to achieve the maximum efficiency for certain radionuclides and sample types, biological, environmental, etc. This objective is usually achieved by means of applying specific pre-treatments, according to the sample type, and the radionuclide and valence state, or by means of achieving stability for a short period, generally insufficient for its suitable metrological characterization as a reference sample.
Usually, the reference radioactive samples come from aqueous solutions in a weakly acid medium, hydrochloric or nitric acid, with such radioactive concentrations that their measurement does not require incorporating great volumes of solution to the scintillation cocktail. However, such reference samples must be subjected to very accurate verifications, controls, and measurements for relatively long periods, up to several weeks, which can rarely be achieved by means of simple, direct incorporation of the radioactive solution to the cocktails normally used at present, unless a careful study of the pre-treatment specifically required for each radionuclide or sample type in question is carried out, such as silicone application to the vials, addition of stable carriers, supplementary addition of an acid solution, saturation of the walls of the vial, etc., as disclosed in different scientific articles and publications (Rodriguez et al., 1993; Rodriguez et al. 1995; Los Arcos et al. 1995; Rodriguez et al. 1996; Ratel 2003).
On the other hand, the most common cocktails at present are of industrial origin and their production batches have insufficient reproducibility of the purity or proportions of their components in the mixture for metrological accuracy purposes. In many cases, the poorly-controllable perturbation introduced by the pre-treatments in the original cocktail composition and which depends on the radionuclide in question, is added to this. Although these variations can be acceptable for routine measurements, they are intolerable for an accurate characterization of reference samples of the several tens of commonly used radionuclides, especially when it is necessary or convenient to apply calculations which take into account the detailed scintillation cocktail composition to assess parameters such as gamma absorption, ionization quenching and other factors depending on this composition.
Accordingly, although the currently available cocktails generally provide a good immediate counting efficiency, they do not ensure a priori sufficient stability, for several weeks, of the samples prepared by simple, direct addition and furthermore their composition and purity is subject to the variability of the industrial production processes and to the required sample pre-treatment itself. These difficulties constrain or even invalidate their use for an accurate characterization, for a sufficient time, of radioactive reference samples of the several tens of commonly used radionuclides.